The present invention relates to a fieldbus network and, more particularly, to a current limiter to protect a fieldbus network from an electrical short in a spur cable or a device attached to a spur cable.
In a typical industrial plant application, sensors measure position, motion, pressure, temperature, flow, and other parameters related to the operation of process machinery and activities. Actuators, such as valves and motor controllers, control the operation of the machinery and process activities. The sensors and actuators are remotely located from the human and computerized controllers which gather information from the sensors and direct operation of the actuators. A communication network links the controllers with the sensors and actuators located in the field.
Heretofore, communication between controllers, remote sensors, and actuators in industrial applications has been by means of analog signaling. The prevailing standard for analog networking of field devices and the control room in industrial applications has been the Instrument Society of America standard, ISA S50.1. This ISA standard provides for a two-wire connection between the controller and each field device. One wire of the system carries the analog signal between the remote device and the controller. The analog signal may be converted to a digital signal useful to a computerized controller. The second wire of the circuit supplies DC power for operation of the remote sensor or actuator.
Communication utilizing digital signaling reduces the susceptibility of the communication system to noise and provides a capability for conveying a wide range of information over the communication network. Digital communication also permits several different devices to communicate over a single pair of wires. Remote devices used in connection with a digital communication system typically incorporate local xe2x80x9cintelligence.xe2x80x9d This permits sensors and actuators to perform diagnostic, control, and maintenance functions locally Further, the local intelligence permits the devices to communicate directly with each other and perform some functions without the necessity of involving a central control facility, thus promoting the development of distributed control systems.
Fieldbus is a generic term used to describe a digital, bidirectional, multidrop, serial communication network for connecting isolated field devices, such as controllers, actuators and sensors, in industrial applications. One such fieldbus is defined by the Instrument Society of America standard, ISA SP50.02. This system utilizes a two-wire bus to provide simultaneous digital communication and DC power to remotely located devices.
While fieldbus installations are as varied as the industrial applications with which they are used, an exemplary fieldbus installation is illustrated in FIG. 1. A twisted pair cable, referred to as the home run 2, connects a digital control system 4 and a DC power supply 6 with a number of devices 8 (actuators, sensors, power supplies, and local controllers) in the field. The digital control system 4 and the DC power supply 6 may be located in a control room 10. The power supply 6 could be located in the field or at a marshaling panel. If wiring runs are long, it may be desirable to power the network from more than one point with additional power supplies 15. A power conditioner 22 is necessary to isolate the DC power supplies from the bus. The DC power supply will attempt to maintain a constant output voltage which, in the absence of isolation, would prevent propagation of the digital signal on the network. The development of the digital fieldbus may also mean that controllers are located in the field.
Several devices 8 can be connected to the home run 2 by spur cables 14 at a terminal referred to as a chicken foot 12 which incorporates signal termination for the home run. A terminator 16 comprising a resistor 18 and a series capacitor 20 connected across the wires of the home run cable 2 must be provided at both ends of the home run cable 2. The varying voltage of the digital signal is produced when an attached device varies the current drawn from the network producing a voltage drop across the resistor 18 of the terminator 16. The capacitor 20 of the terminator 16 prevents dissipation of the DC power through the terminator resistor 18 while permitting transmission of the high frequency digital signal on the bus. In addition, the terminators 16 serve to prevent signals from reflecting from the ends of the home run wires 2.
In addition to the devices connected to the home wiring at a chicken foot, devices can be connected along the home run cable 2 with spur cables 14 that are connected to the home run by spur connectors 13. The chicken foot 12 and the spur connectors 13 provide a convenient means for interconnecting the wires of the home run 2 and the spur cables 14. Heretofore, the positive and negative wires and the shield of the spur cable 14 have been directly connected to the corresponding conductors of the home run cable 2. Direct connection of the individual wires is facilitated by terminals within the chicken foot 2 and spur connection 13 connector blocks. However, if the wiring of a spur cable 14 or a connected device should become shorted, neither the DC power nor the digital signals can be sent over the network and the entire network is disabled. Disabling the network may cause an entire plant or process to be shut down with severe economic consequences. Further, safety may make the ability to continue to monitor and control other parts of the plant or process particularly essential when one part of the system is malfunctioning. The shutting down of the entire network may also make it much more difficult and time consuming to find the short and make repairs.
What is desired, therefore, is an apparatus that causes a network spur to appear as a high impedance in the event of a short circuit in the spur, limiting the current drawn by the spur and permitting the remainder of the network to continue to function. Further, an indicator of abnormal current draw in a spur is desired to facilitate maintenance and repair.
The present invention overcomes the aforementioned drawbacks of the prior art by providing a fieldbus network comprising a home run conductor; a spur conductor electrically connected to the home run conductor; and a spur current limiter interposed between the spur conductor and the home run conductor. The spur current limiter provides a conduction path between the spur conductor and the home run conductor in which the impedance is varied as a function of the current in the spur conductor. In the event of a short circuit in the wiring of the spur cable or a device attached to the spur cable, the current in the spur conductor will increase causing the impedance of the current limiter to increase. The increased impedance of the conduction path through the current limiter limits the current flow in the spur conductor. During current limiting operation, the current limiter causes the spur to appear as a large impedance so the remainder of the network can continue to function.
A method of connecting a spur cable to a home run of a field bus network is provided comprising connecting a home run conductor to a connecting conductor in a connecting block; connecting a spur cable conductor to an electrically conducting current limiter; and engaging the connecting block and the connecting conductor with the current limiter to electrically connect the spur conductor and the home run conductor. The current requirements of a spur may not be known in advance. Connecting spurs to the home run through a separate current limiter with a plug connector that engages the connecting block permits the use of a universal connecting block in assembling networks with a current limiter selected from a stock for each specific spur""s operating current requirement.
The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.